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Due to the rainy weather over the past two weeks, we are still planting full-season soybean in some areas. In addition, it appears that wheat harvest is not far off (some wheat at the Tidewater AREC was at 23% moisture today!). So, should we be increasing our seeding rates?

In general, yes. But, big increases probably will not be needed until late-June. Below are some seeding rate data that we collected from soybean planted in early-June after barley. First, we don’t have a lot of data of soybean grown after barley, so I don’t have as much confidence in the exact seeding rate needed. Note that there is a wide range in the optimal seeding rates, illustrated by the area between the dotted lines in the graph. Although, these data may not directly apply to full-season soybean (no small grain), it should be close.

I think that we should now be using 120,000 to 160,000 seed/acre. The range will depend on the planting date. In general, I’d suggest bumping up your seeding by 20-30,000 seed/acre per week through June.

If you remember the seeding rate data that I shared in this blog last month for May-planted soybean (see Soybean Seeding Rates – How Low Can We Go?), I stated that maximum yields could be obtained with only 95,000 to 110,000 seed/acre when the yield potential is greater than 40 bushels/acre. That’s pretty low, but was adequate for maximum yield under good growth conditions. For less than 40 bushel potential, seeding rates needed to be a little higher. In the above graph, it appears that more seed is needed to obtain 55 to 70 bushels/acre after barley, I cannot fully explain why; therefore, I would assume that this response is primarily due to the location that we obtained the data (again, we don’t have a lot of data).

Once we get into mid- to late-June, I’d rather see a seeding rate of 180,000 to 220,000 seed/acre, depending on planting date. This is based on the data to the right. You’ll notice that, like full-season soybean, the optimal seeding rate falls with greater yields. This is most likely due to greater leaf area with those high-yielding locations. As I’ve stated often, the seeding rate response can usually be traced back to whether or not the crop developed enough leaf area to capture 90-95% of the light by early pod development. Unfortunately, I don’t have any double-crop data planted following wheat with yields greater than 55 bushels/acre. I hope to solve that problem this year with new experiments.

It seems that everything that you read about soybean seeding rates is that we are planting too many seed. In general, I agree – at least for full-season soybean. We still seem to have that mindset that it takes 1 bag of seed per acre. But many of you have been listening and are taking the seeding rate down to 100,000 to 120,000 seed per acre, with no noticeable difference in yield. But, can you go lower? And how low can you go?

To help answer this question, we have re-analyzed about 10 years of data that we collected from dozens of experiments conducted from 2003 through 2011. But instead of just looking at average yield response, we separated these responses into soybean yield potentials.

Why did we do this? I’ve always thought that more seed is needed to maximize yield on low-yielding fields (or portions of fields) and less seed are needed or high-yielding fields or portions of fields. Yes, this means that I’m asking you to spend more money on the least profitable fields and less money on the most profitable fields. Still, this strategy will likely be more profitable over all acres.

There are a few things worth noting about the graphs to the left. First, I’ve separated the data into low (20-40 Bu/A), medium (40-55 Bu/A), and high (55-70 Bu/A) groupings. We decided on these levels by analyzing the data over and over at many different yield levels. The resulting three levels were most stable and predictive.

Second, we used two statistical methods to fit a curve to the data to intentionally give us a range of seeding rates needed to maximize yield. This allow us to recognize the variability in the data and reflects our confidence in the response. Pay particular attention to the wide range of seeding rates necessary to maximize yield at the 20 to 35 bushel yield potential. This reflects the yield variability and the variability in the response of yield to seeding rates that are common in low-yielding years or fields. We just are not as confident in this set of data. Some years or locations, we could get by with 100,000 seed/acre; in others, it took more than 140,000. With the other yield potentials, the range is pretty tight. In other words, I have more confidence in recommending 110,000 or even less than 100,000 seed/acre in these instances.

Finally, we see that it takes, in general, less seed at high yields – which verifies my earlier statement that less seed are needed for higher yield potentials.

So what have I settled on? Below are my suggestions.

But, you may ask, “What about yields greater than 70 bushels per acre? That’s a good question. But, I cannot answer it confidently since we have little data in that range. We are however conducting new experiments this summer to update our data.

But until that data is available, here are my thoughts. I think that lower seeding rates will work until you get to the 100+ bushel yield range. After that, I suspect that we are running low on reproductive nodes (node on the plant where pods can form). For instance, if we only have 80,000 plants/acre, we would need 15 reproductive nodes containing 4 pods on every one of these nodes! While this is possible, I’m assuming in this calculation that we will grow 2,500 seed/pound and 3.0 seed/pod. Taking that down to a more normal 2800 seed/pound and 2.5 seed/pod, that means we need 6 pods per node! I think that we are starting the expect a little much from single plant in this case.

So, for 100+ bushel yield environments, I’d suggest to gradually increase your seeding rate from the ones suggested above. I do understand that we have very few 100+ bushels fields, but I have seen parts of the field exceeding this when I’m watching a yield monitor. An we commonly have plots within our small-plot tests exceeding 90 and 100 bushels.

Finally, am I suggesting that we may be able to vary our soybean seeding rate as we do corn? Yes, I’m suggesting that. We will be validating some variable-rate-seeding (VRS) on two farmer’s fields this year. If you know of anyone who has VRS planters and who would like to participate in an on-farm test, let me know.